1) Is your Zyxel P600 in router mode?
2) What are the required ports you need to open for the DVR?
3) Is your ISP connection on Static IP/PPPoE/Dynamic IP/etc?

Plug in your modem to the PC first and establish an Internet connection. Once you have connected, then go to the prompt and do IPCONFIG /ALL and take note of the IP Address, Subnet, Gateway and DNS values you have.

#1 If your Modem is in bridge mode (i.e. your IP address is not 192.168.xxx.yyy), then you can just configure the WAN port on the Belkin as usual with Dynamic IP. Take note of how the Belkin is configured for its LAN side. From there, assign a Static LAN IP to the DVR which is in the same network as the Belkin (i.e. 192.168.xxx.yyy) and do the necessary port forwarding to the DVR's LAN IP from the Belkin router interface.

#2 If your Modem is in router mode (i.e. your PC gets a 192.168.xxx.yyy IP address when directly connected), then you must first turn it to bridge mode so it would be easier for you to manage the pot forwarding from the Belkin only. Else. you'll have to do port-forwarding to the WAN port of the Belkin router from the Zyxel.

1) Is your Zyxel P600 in router mode?
2) What are the required ports you need to open for the DVR?
3) Is your ISP connection on Static IP/PPPoE/Dynamic IP/etc?

Plug in your modem to the PC first and establish an Internet connection. Once you have connected, then go to the prompt and do IPCONFIG /ALL and take note of the IP Address, Subnet, Gateway and DNS values you have.

#1 If your Modem is in bridge mode (i.e. your IP address is not 192.168.xxx.yyy), then you can just configure the WAN port on the Belkin as usual with Dynamic IP. Take note of how the Belkin is configured for its LAN side. From there, assign a Static LAN IP to the DVR which is in the same network as the Belkin (i.e. 192.168.xxx.yyy) and do the necessary port forwarding to the DVR's LAN IP from the Belkin router interface.

#2 If your Modem is in router mode (i.e. your PC gets a 192.168.xxx.yyy IP address when directly connected), then you must first turn it to bridge mode so it would be easier for you to manage the pot forwarding from the Belkin only. Else. you'll have to do port-forwarding to the WAN port of the Belkin router from the Zyxel.